In bulk semiconductors, carriers are free to move in three dimensions as their movement is not restricted by potential wells. However, interesting and useful effects arise if semiconductor structures are fabricated in which carriers have their movement restricted so that they can move in less than three dimensions. For example, heterojunctions have been fabricated with a doped wide bandgap material adjacent to an undoped narrow bandgap material. This type of structure is commonly referred to as modulation doping and has increased carrier mobility, as compared to doped bulk semiconductors, because the carriers are essentially confined to the undoped material where they have high mobility because of the reduced impurity scattering. Modulation doping is useful in field effect transistors and such transistors are commonly referred to as selectively doped heterojunction transistors (SDHT).
Additional and interesting effects may arise if the carrier motion is further confined to dimensions which are sufficiently small so that quantum effects become important. Structures demonstrating such effects are commonly referred to as quantum wells. Two dimensional quantum well structures were first demonstrated in the GaAs/GaAlAs materials system. In these quantum well structures, the carriers are free to move in two dimensions, but quantum effects are significant in the third dimension. That is, the energy levels are quantized in one dimension but form continua in the other two dimensions. Such quantum well structures are of interest because, for example, the emission frequency of a double heterostructure laser is shifted from that expected for bulk semiconductors due to the change in allowable energy levels caused by the presence of quantum effects.
The success of devices using two dimensional carrier confinement and their technological and commercial significance has led to desires for structures in which carrier movement is further restricted, i.e., structures in which carriers can move freely in one dimension (1 dimensional confinement) or in which their positions are essentially localized. It is hypothesized by those skilled in the art that quantum confinement to one and zero degrees of freedom may lead to new physical phenomena with interesting device applications, and attempts have been made to fabricate structures in which carriers are so confined. One such attempt to fabricate a one dimensional quantum well structure is described in Applied Physics Letters, 41, p. 635, 1982. For several proposals, see Japanese Journal of Applied Physics, 19, p. L735, 1980, and Applied Physics Letters, 47, p. 1325, 1985.
However, these proposals and attempts have not yet achieved the success that has been obtained with two dimensional quantum well structures. This relative lack of success undoubtedly has its origins in the technological difficulties associated with the fabrication of structures having, in two or three directions, dimensions which are close to the carrier de Broglie wavelengths. These wavelengths are typically less than approximately 500 Angstroms.
Structures have been fabricated having a plurality of quantum wells which are typically spaced from each other along a line perpendicular to the major surfaces of the quantum wells. These structures are, not surprisingly, commonly referred to as multiquantum well structures. Due to the periodicity of this structure, which differs from that of the underlying crystal lattice, it is sometimes referred to as a superlattice. However, the layers of a superlattice are frequently large and quantum effects are not significant.
Attempts have been made to fabricate small features in superlattices by implanting impurities to introduce compositional disordering. See, Japanese Journal of Applied Physics, 24, pp L516-518, July, 1985. Other studies have been performed on interdiffusion in superlattices after ion implantation.
Structures in which the carriers are confined so that they have essentially only a single degree of freedom are conveniently termed quantum well wires (QWW). By analogy, structures which restrict the carriers so that they have no degree of freedom can be aptly termed quantum well boxes (QWB).